Model your nitrogen to account for in-crop mineralisation
“Mineralisation can provide somewhere up to 120 kilograms of N per season, depending on a number of conditions, with the rate varying across the season, so it isn’t something that’s easy to calculate. Overestimating mineralisation or not taking it into account can result in either too much or not enough fertiliser N being applied,” he said.
Mineralisation depends strongly on temperature, moisture and soil organic matter. During winter in southern cropping regions, mineralisation is low because of cool temperatures, with the rate picking up in spring as the temperature increases. However, the highest crop demand is generally at the end of winter.
The rate of mineralisation cannot be effectively measured in the field for two reasons – the crop is taking up nitrogen as soon as it mineralises, so it is challenging to measure the input of plant available nitrogen through mineralisation, and the process of sampling to take measurements disturbs the soil and stimulates microbes, which changes the result.
“A practical maximum for mineralisation in a moist soil containing 1% organic carbon in spring is around 1kg per hectare per day, while a really strong crop will be using up to 5kg/ha/day at the same time. Because of this, it is easy to write-off in-crop mineralisation, as it doesn’t provide enough N to provide the entire nutritional requirements for the plant,” Dr Angus said.
However, there are potentially negative consequences for both under and over-application of nitrogen.
“If there’s not enough nitrogen applied to the crop, obviously the growth will not meet the yield potential. However applying too much nitrogen can also reduce crop yield and quality, as well as costing more. Excess nitrogen reduces soluble carbohydrate in the stem at flowering, as the two have an inverse relationship” he said.
“This means that if there is moisture stress late in the season, the plant won’t have adequate soluble carbohydrates to fall back on for grain filling and can start haying off. This is a risk for farms in the path of an El Niño, as forecast for 2015.”
Using the models provided by CSIRO takes into account soil test results, crop conditions and weather forecasts to provide recommendations on optimal nitrogen application rates.
Can in-crop mineralisation be improved?
Cultivation does not significantly affect mineralisation in Australian soils, unlike North American soils.
“In Australia we cultivate much shallower than they do in North America, so there’s very little difference in our soils between cultivating and no-till, in this aspect. The one thing that should theoretically help is stubble retention, which increases soil moisture, so should increase mineralisation, but we have so far been unable to find a measurable effect,” he said.
Break crops can help. Legume crops fix nitrogen to give a higher mineral N at the time of sowing in the following year and during crop growth (see “Where to get your N”), and perennial pasture residues have been shown to lead to higher rates of N mineralisation in the crop following the break.
Dr John Angus, CSIRO, email@example.com
CSIRO Yield and N calculators.
GRDC Project Code CSO00198, CSO00209, CSO00212, MF00001, DAV00095